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Gaseous State: Kinetic Molecular Theory

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Gaseous State: Kinetic Molecular Theory - Lesson Summary

In order to understand the behavior of gases at the molecular level, scientists developed a theoretical model called Kinetic Molecular Theory (KMT) or Microscopic Model of Gases.

Kinetic Molecular Theory assumptions:

a) Gases consist of very large number of extremely small particles called molecules, which are in constant, continuous, random and straight-line motion.

b) During their motion, they collide with each other and against the walls of the container. The pressure exerted by the gas is due to the bombardment of its molecules on the walls of the container.

c) The molecules of a gas are separated from each other by great distances .Hence the actual volume of all the molecules of the gas is negligible when compared to the total volume occupied by the gas.

d) Attractive and repulsive forces between the molecules of a gas are negligible as they are much away from each other.

As gases expand and occupy the entire space available to them, because of weak intermolecular forces of attraction.

Thus attractive or repulsive forces between the molecules of a gas are negligible.

Individual molecules do not gain or lose energy as a result of collision. Therefore, collision between molecules is perfectly elastic.

The average kinetic energy of the molecules is proportional to absolute temperature.

When the temperature of a gas is raised, the molecules start moving faster, as their individual kinetic energies increase and also average kinetic energy increases.

Based on these, an equation for pressure of a gas is derived as

Kinetic Gas Equation     
                                 PV = 1/3 . mNu2

Where N = Total number of molecules in volume V
                 m = Mass of the gas molecule
                 u = Root mean square velocity of the molecule

According to Kinetic Molecular Theory, pressure exerted by a gas is due to molecular collisions on the walls of the container. As the number of collisions increases, the pressure of the gas also increases.

The average velocity of molecules is constant at a given temperature.

When the volume of a gas is decreased, the space available for the movement of molecules decreases. Then the number of collisions on the walls of the container increases resulting in an increase in the pressure of the gas.

According to Boyle's law 'as the volume of a gas is decreased, pressure is increased for a given mass of a gas at a given temperature'.

According to Charles's law 'at constant pressure, when the temperature of a given mass of a gas is increased, its volume increases'. According to Avogadro's Law 'equal volumes of all gases under similar conditions of temperature and pressure contain an equal number of molecules'.


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